1. Field of the Invention
The present invention relates to a zoom lens assembly for use in a photographic camera, a video camera or the like. The present invention relates more particularly to a zoom lens assembly capable of extending a movable barrel a substantial distance.
2. Prior Art
Cameras, including compact cameras and 8 mm video cameras, provided with zoom lens assemblies are widely used. In this type of cameras, the movable barrel of the zoom lens assembly is adapted to retract into the camera body until the movable barrel does not protrude from the camera body when the camera is not in use, so as to facilitate carrying and storing the camera. Recently, cameras with zoom lenses have been given a larger zooming ratio by increasing the maximum stroke or extension of the movable barrel. It is also desirable for such a zoom lens assembly having a large zoom ratio to be compact in the unused state.
FIG. 9 of the accompanying drawings shows an example of a conventional zoom lens assembly having a double helicoid mechanism. A camera body 9 is assembled fixedly with a stationary barrel which is formed with an internal helicoidal thread 11a, and a front cover 100 is secured to the camera body 9 so as to cover the stationary barrel 11. The internal helicoidal thread 11a engages with an external helicoidal thread 12a formed on a peripheral part of a middle barrel 12. The middle barrel 12 is also formed with a gear portion 12b through which the middle barrel 12 is rotated by a motor 13. The length of the gear portion 12b of the middle barrel 12 in the direction of the optical axis P of the lens assembly is approximately equal to the maximum amount of axial movement of the middle barrel 12.
The middle barrel 12 is further formed with an internal helicoidal thread 12c, which engages with an external helicoidal thread 16a formed on a peripheral part of a movable barrel 16. A first lens group 14 and a second lens group 15 constituting the optical system of the zoom lens assembly are assembled in the movable barrel 16. When the middle barrel 12 rotates, the movable barrel 16 moves axially forward and backward parallel to the optical axis of the lens assembly, while guided along a straight movement guide member 17. A protection barrel 18 cover the gear portion 12b of the middle barrel 12 is provided because the gear portion 12b would otherwise be outwardly exposed when the movable barrel 16 protrudes from the front cover 10.
Because the gear portion 12b of the middle barrel 12 has an axial length approximately equal to the maximum axial movement of the middle barrel 12, the production cost of the above-described conventional zoom lens assembly is high. Furthermore, the need for the protection barrel 18 increases the external diameter of the middle barrel 12 by an amount corresponding to the thickness of the protection barrel 18. Because the movable barrel 16 is guided along the straight movement guide member 17, it is necessary to elongate the straight movement guide member 17 in order to increase the maximum stroke of the movable barrel 16. However, such an elongated straight movement guide member 17 would project from the front end of the movable barrel 16 when the movable barrel 16 moves back into a retracted position, unless the stationary barrel 11 is also elongated so as to cover the front end of the straight guide member 17. But such an elongated stationary barrel would enlarge the camera body.
In order to solve the above problem, a zoom assembly has been suggested, for example in Japanese Laid-Open Patent Appln. No. 1-306808, wherein a lens guide ring, which corresponds to the straight guide member, is movable along with a cam ring so as to reduce the axial length of a stationary barrel. Because a cam surface formed on the cam ring will be exposed to the image-forming portion of the zoom lens assembly when the cam ring moves outwardly, a cover barrel is provided for covering the cam surface. Accordingly, although the axial length may be reduced, the diameter of the stationary barrel must be increased by the cover barrel.